Passerine birds are the most diverse and abundant group of birds. Containing the majority of living bird species, they inhabit every continent on Earth except Antarctica. What unites them is their tendency to perch and a body form to match. Passerine birds have three toes forward and one backward, which allows them to hold onto perches. Whether perching on branches, cliffs, rocks, fences, or other surfaces, rough skin on the bottom of the foot provides traction. A tendon on the back of each leg tightens to bend the bird's foot so that it curls around the perch. Many passerines spend the night on perches without falling off. Stiff tail feathers help them balance on vertical perches such as tree trunks. Their ability to perch makes a huge range of habitats available to them. A robin in New York City and a bird of paradise in a forest in New Guinea are both passerines.

Owls are expert night hunters of birds, lizards, insects, mice, or even fish. Various adaptations make them skillful at catching prey in low light. The feathers across the front of their faces are flattened into a disk, surrounded by a fluffy ruff, which channels sound toward the ears. In many owls, uneven placement of the ears on their skull allows them to pinpoint the location of sounds with extreme accuracy. Owl eyes have extra light-sensing cells (rods) packing into tubular eyeballs that extend back into their heads. The eyeballs cannot move in their sockets, but owls instead swivel their necks more than 180 degrees to see what is behind them. Fringed, soft feathers on their wingtips act as mufflers, making for silent flight as owls sneak up on prey. Owls regurgitate parts of their meals, spitting out a pellet made of fur, feathers, bones, or other indigestible body parts.